The present invention relates to aluminum smelting and, more particularly, to the art of winning aluminum metal from Al.sub.2 O.sub.3.
Conventional Hall-Heroult-type aluminum smelting cells employ a molten aluminum metal pad as a cathode and, resting on the molten pad, essentially a cryolite (Na.sub.3 AlF.sub.6) electrolyte bath to which aluminum fluoride is added to reduce the weight ratio of NaF to AlF.sub.3 (the bath ratio) to a range of greater than 1.1:1 and up to 1.3:1, thereby improving the current efficiency at operating temperatures around 970.degree.C. However, attempts at operating at progressively lower bath ratios have been frustrated by the forming of a crust of frozen electrolyte over the molten aluminum pad cathode as electrolysis proceeds. This crust causes deposition of sodium, thus harming current efficiency, drastically increases resistance at the cathode, and reduces metal coalescence to the point that a cell can no longer be operated.